The present invention generally relates to actuators for valves, for example external actuators for valves such as butterfly valves.
Actuators for valves, such as butterfly valves, are well known. In some arrangements, a drive screw is rotated to cause a traveling nut to move a crank arm attached to the valve shaft to rotate the valve member into and out of engagement with the valve seat.
In the case of some butterfly valves, the valve member may be rotated approximately 90 degrees between an open position and a closed position. The precise closed position may change over time as the valve seat wears, or as conditions require greater or lesser closing force of the valve member against the valve seat. Typically some type of abutment occurs between the traveling nut and a fixed nut carried on the drive screw, as shown in U.S. Pat. Nos. Re. 29,253 and 3,147,766, or between the crank arm and an adjustable stop, such as a stop screw projecting through a wall of the actuator body, as shown in U.S. Pat. No. 3,385,120.
The drive screw may be supported and held in place through use of a single locking collar secured about the circumference of the drive screw near its penetration into the housing on a screw insertion end. The collar may be held on the drive screw through use of a locking pin so that it rotates with the drive screw. The housing may define a cylindrical holding chamber for rotatably holding the collar, which chamber may also be partially defined by a removable access cover. Removal of the access cover provides for insertion and removal of the drive shaft. Thrust bearings may be on one or both of the axial ends of the collar for rotatable engagement between the collar and surfaces of the holding chamber.
In designs that utilize stop screws projecting through the actuator body, as well as some other designs, known configurations risk disadvantageous deformation of the drive screw as a result of loads placed on it. For example, when the drive screw is rotated to urge the valve member into a fully open or closed position, the traveling nut may be urged against the stop screw. In the case of some valve actuators, with an example being relatively large valve actuators, a considerable closing force may be imparted onto the drive screw. It is not uncommon, for example, for a load of 450 ft-lbs to be applied in some large butterfly valve actuators. This load is, at least to some extent, distributed through the thrust bearings provided in the single locking collar on the screw insertion end. Some valve actuators of the prior art have suffered a shortened service life as a result of these loads. With the traveling nut located along the drive screw some distance from the single locking collar and its thrust bearings, for example, some valve actuator drive screws of the prior art have bowed or otherwise deformed under such loads.